Home battery backup systems

[SpaceBus]

Moderator moved from Going Solar! thread to new thread for discussion

Total novice here, but I have a few questions. My water heater is rated at 18kw max and I'm trying to see if it were even possible to run it off of a battery bank. Does this means I should get at least an 18kw inverter for those times the water heater calls for max amperage (2 x 40 amp DP breakers on a 200 amp service box)?

 

[jetsam]

Total novice here, but I have a few questions. My water heater is rated at 18kw max and I'm trying to see if it were even possible to run it off of a battery bank. Does this means I should get at least an 18kw inverter for those times the water heater calls for max amperage (2 x 40 amp DP breakers on a 200 amp service box)?

1) Do you want to go shut off the water heater breakers every time you use the microwave? Of course not, so size the inverter to the total load it may need to supply. When you run the dryer, the boiler and the refrigerator may kick on without warning, etc. You can reduce this by planning to not use some circuits when off the grid (I shut off my electric dryer breaker so nobody tries to use it, and thus don't need to include it in my capacity planning).

2) 18kw is crazy for a DHW heater unless it's commercial (or some kind of tankless heater).
Is it a boiler that also does DHW?

 

[begreen]

On demand heater?

 

[SpaceBus]

1) Do you want to go shut off the water heater breakers every time you use the microwave? Of course not, so size the inverter to the total load it may need to supply. When you run the dryer, the boiler and the refrigerator may kick on without warning, etc. You can reduce this by planning to not use some circuits when off the grid (I shut off my electric dryer breaker so nobody tries to use it, and thus don't need to include it in my capacity planning).

2) 18kw is crazy for a DHW heater unless it's commercial (or some kind of tankless heater).
Is it a boiler that also does DHW?

On demand heater?

Yes, it is a small on demand water heater. Usually it doesn't pull max load since I have a 40 gallon tank heated by the cookstove (and already existing rooftop collector in the future).

 

[jetsam]

I've never even seen an electric tankless one. They're almost unheard of around here, though you do rarely see natural gas tankless (mostly as point of use heaters).

I had a natural gas unit decades ago overseas, and it did a great job supplying a small house. (It was located in the bathroom, which was a bit odd.)

 

[SpaceBus]

I've never even seen an electric tankless one. They're almost unheard of around here, though you do rarely see natural gas tankless (mostly as point of use heaters).

I had a natural gas unit decades ago overseas, and it did a great job supplying a small house. (It was located in the bathroom, which was a bit odd.)

Mine is an Ecosmart ECO 18. It works very well and modulates the heating elements based on flow and incoming water temperature. In my case the incoming water is 75-140df since it is coming from a 40 gallon tank heated by my cookstove. The water heater is set at 130df and then feeds into my pex manifold.

I have found a 20kw charger inverter, but it does not appear to be UL listed and probably isn't approved for a grid tie in. Most of the 20 kw inverters I've seen are ul approved and grid tie in approved, but require 200 - 1000v feed. That's a heck of a battery bank if I want grid tied with storage.

Sorry to thread jack, just trying to pick the brains of folks that have done this. We didn't want to rely on propane, but I didn't really think about how to feed the water heater from a battery bank.

Edit: I'm better off just running a 7600w 40 amp charger inverter and hope I never need more than 40 amps at the electric water heater. Trying to make a 20kw inverter play nice with a residential setup will be impossible.

 

[begreen]

Most of the 20 kw inverters I've seen are ul approved and grid tie in approved, but require 200 - 1000v feed. That's a heck of a battery bank if I want grid tied with storage.

Not with a DIY bank based on a Prius, Volt or Tesla battery. The Volt is ideal as it can be broken down into 48v modules. That said, a 40amp load on a battery bank is going to hit it hard.

 

[SpaceBus]

Not with a DIY bank based on a Prius, Volt or Tesla battery. The Volt is ideal as it can be broken down into 48v modules. That said, a 40amp load on a battery bank is going to hit it hard.

The water heater is nice because it can modulate a single, or all three if needed, 6000 watt heating element multiple times a second. Unless something crazy happens I should never be heating the water from anything less than 55 df. I haven't seen input temps below 75df and I frequently check when my wife is running taps out of curiosity. We keep the house above 70df for the most part, so the tank should always stay warm. Would an intermittent 40 amp hit on a system made up of several lead acid batteries in series and parallel? Would lithium batteries really be worth the huge premium from this perspective?

The biggest issue I see trying to make a 20kw inverter work is getting a 3phase system to place nice with my residential service, which doesn't seem possible. I would have to go off grid and put a 400 amp panel in my house; or build an outbuilding with a 400 amp panel and feed my house with an interlink between the meter and the house panel from an 80 amp breaker on the 400 amp outbuilding panel. Either way sounds crazy expensive and I'm better off keeping the range boiler above 70df if I want to build a solar system for my house.

 

[begreen]

Used Volt batteries are much more affordable and majorly less maintenance. I wouldn't use lead-acid for more than a 48VDC system and I would use them for high amperage resistance heating of water or air.

 

[jetsam]

The water heater is nice because it can modulate a single, or all three if needed, 6000 watt heating element multiple times a second. Unless something crazy happens I should never be heating the water from anything less than 55 df. I haven't seen input temps below 75df and I frequently check when my wife is running taps out of curiosity. We keep the house above 70df for the most part, so the tank should always stay warm. Would an intermittent 40 amp hit on a system made up of several lead acid batteries in series and parallel? Would lithium batteries really be worth the huge premium from this perspective?

The biggest issue I see trying to make a 20kw inverter work is getting a 3phase system to place nice with my residential service, which doesn't seem possible. I would have to go off grid and put a 400 amp panel in my house; or build an outbuilding with a 400 amp panel and feed my house with an interlink between the meter and the house panel from an 80 amp breaker on the 400 amp outbuilding panel. Either way sounds crazy expensive and I'm better off keeping the range boiler above 70df if I want to build a solar system for my house.

I bet your water heater has 4 internal heating elements, and I further bet that each one is either already fused/breakered, or that at least they are each designed to be replaceable. Either way, it shouldn't be a major project to shut off 2 of them (actually, figure out which leg the circuit board runs off of, and just shut off the opposite double 40, all done).

I don't know if it'd be smart enough to use the 2 elements that have power at times of low draw, though maybe you'll get lucky and the 2 legs the circuit board is on are the "low" heat elements.

 

[SpaceBus]

I bet your water heater has 4 internal heating elements, and I further bet that each one is either already fused/breakered, or that at least they are each designed to be replaceable. Either way, it shouldn't be a major project to shut off 2 of them (actually, figure out which leg the circuit board runs off of, and just shut off the opposite double 40, all done).

I don't know if it'd be smart enough to use the 2 elements that have power at times of low draw, though maybe you'll get lucky and the 2 legs the circuit board is on are the "low" heat elements.

It won't work if one breaker is switched or disconnected, unfortunately. When we make the jump to solar I'll most likely have a shed/barn that all the equipment lives in and I'll set up a line side tap and just feed my service panel in the house directly with a 20kw inverter.

 

[begreen]

DIY power wall.

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[peakbagger]

DIY power wall.

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Maybe I am missing something but no mention of a BMS system in the video. My limited understanding of LI battery packs is Lithium Ion batteries need to have a BMS for each cell. If they don't, bad things can happen during charging like a fire and toxic fumes.

Most of the discussions on various Solar Power Forums regarding reusing Lithium Ion battery packs seem to end up at what BMS do they use? I haven't seen a lot of agreement and the various BMS suppliers are gladly willing to sell them but not a lot of information on how to make sure they operate reliably enough not to allow a cell to overcharge. The theory is if one cell tries to overcharge, the BMS cuts that cell out of the charge circuit. That equalizes the string of batteries maximizing state of charge. There are companies that make lithium ion retrofits for golf carts that include a BMS so I know its possible.

 

[begreen]

Maybe I am missing something but no mention of a BMS system in the video. My limited understanding of LI battery packs is Lithium Ion batteries need to have a BMS for each cell. If they don't, bad things can happen during charging like a fire and toxic fumes.

Most of the discussions on various Solar Power Forums regarding reusing Lithium Ion battery packs seem to end up at what BMS do they use? I haven't seen a lot of agreement and the various BMS suppliers are gladly willing to sell them but not a lot of information on how to make sure they operate reliably enough not to allow a cell to overcharge. The theory is if one cell tries to overcharge, the BMS cuts that cell out of the charge circuit. That equalizes the string of batteries maximizing state of charge. There are companies that make lithium ion retrofits for golf carts that include a BMS so I know its possible.

He has a lot of videos including successful BMS systems and not so successful

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[peakbagger]

Okay I just want to make sure folks do not think rigging up a battery is as easy as rigging up Lithium Ion cells. In order to get the performance out of them they need a BMS and special charging whch is the tricky part. With the amperage involved, minor mistakes lead to real large arcs.

 

[begreen]

Indeed, one needs to treat a battery bank of this size with great respect. He's been experimenting and refining his powerwalls from Volt cells for a few years now. I like that he documents his successes and failures well. I think his first series with a Gen 1 battery was over something like 8 or 10 videos.

 

[SpaceBus]

It seems like the Nissan leaf cells work very well, but a quality BMS is essential. From my limited research there aren't any off the shelf units that would work. Perhaps I just haven't found them.

 

[semipro]

Indeed, one needs to treat a battery bank of this size with great respect. He's been experimenting and refining his powerwalls from Volt cells for a few years now. I like that he documents his successes and failures well. I think his first series with a Gen 1 battery was over something like 8 or 10 videos.

I've been following David Poz for a while and I'm impressed with his work.

 

[semipro]

It seems like the Nissan leaf cells work very well, but a quality BMS is essential. From my limited research there aren't any off the shelf units that would work. Perhaps I just haven't found them.

I remember one guy, IIRC somewhere in Scandanavia, that installed old Leaf batteries in an old server rack with a BMS for backup storage. It seemed like a nice setup.
I'll try to find the URL later.

 

[SpaceBus]

I remember one guy, IIRC somewhere in Scandanavia, that installed old Leaf batteries in an old server rack with a BMS for backup storage. It seemed like a nice setup.
I'll try to find the URL later.

The Nissan cells seem by far the most affordable and scalable. I saw a lot for 50 individual modules going for about $3k while looking into the viability of repurposed automotive batteries. Unfortunately I didn't save it, but I did see a lot of 98 modules for a decent price, but it's gone now. Sometimes the rain and clouds can hang around for a week or more here in Maine, so if I designed a system to carry the house for 7-10 days then I'm basically looking at a battery bank that could support my on demand water heater without any preheated water. Well, it would run for a few minutes anyway, but I didn't design the system to run without preheated water.

 

[peakbagger]

The early Leaf batteries have the reputation that they have lost a lot of capacity. Nissan pushed them hard to get more range and the trade off was steeper degradation. Still adequate for home use but not so good for cars.

I have been keeping an eye out for a packaged BMS solution but to date the only ones I find are science experiments. I think it comes down to liability. A big company can cover liability but to a little guy I expect the cost for liability insurance to sell a packaged BMS would be steep as the potential risk is high if a pack lights up and burns a house down. There are couple of vendors on Ebay that sell battery packs with a big warning that a proper BMS is needed. When you look through their other offerings they sort of sell BMS components but info is lacking.

 

[begreen]

Here is a cool video of the final resurection of a Smart For Two battery. Note that in the middle of the video he talks a bit about his home running on the Volt system. This is everything including hot water, refrigeration etc. on a 6kW inverter. They have tripped the breaker a couple times when the HW htr was running and then another heavy load was turned on.

Also, look at the comprehensive links provided below the video. Very helpful. Here is the BMS he used.
(broken link removed to https://www.lithiumbatterypcb.com/product-instructionev-battery-pcb-boardev-battery-pcb-board/ev-battery-pcb-board/smart-bms-of-power-battery/)

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[Starbrightsteve]

With respect to the original post: if you are adding backup power to a solar system, I don’t think an electric resistance device of any kind ought to be connected to the inverter /battery. Our solar backup will handle the refrigerator, freezer, basement sump pump, and a few LED lights and the stereo/radio. If we really need a shower we can use our camper.

 

[maple1]

It would seem to me with everything being talked about here it might be worth ditching the on demand for an ordinary 4500w tank heater. Or maybe even better, a heat pump heater.

 

[begreen]

It would seem to me with everything being talked about here it might be worth ditching the on demand for an ordinary 4500w tank heater. Or maybe even better, a heat pump heater.

Agreed. This makes more sense and the resistance heater in that case could be switched off, say during an extended stormy period. As noted above, Poz ran his house all summer on his 6kW inverter. This could also be done with a Tesla PowerWall if one didn't wnat to DIY.